Improving the TADF properties of deep-blue multiple-resonance emitters by strategic oxygen–sulfur replacement

An oxygen-sulfur replacement for improving thermally activated delayed fluorescence (TADF) materials based on a multiple-resonance (MR) effect is reported. A comprehensive computational analysis of four possible isomers revealed that the precise placement of the sulfur atom is crucial to suppress undesired spectral red-shifts. Among them, a promising deep-blue emitter, DOB2-DABNA-C-NP-S-1, exhibits emission at 458 nm with a narrow full width at half maximum (FWHM) of 20 nm. Moreover, its reverse intersystem crossing rate constant (kRISC) of 2.9 × 106 s-1 is three times larger than that of the oxygen-based analog (9.0 × 105 s-1). An OLED device incorporating DOB2-DABNA-C-NP-S-1 as an emitter achieves ultrapure deep-blue electroluminescence at 461 nm with Commission Internationale de l'Éclairage (CIE) coordinates of (0.133, 0.077), satisfying the blue emitter standards set by the National Television System Committee (NTSC). The device demonstrates outstanding external quantum efficiencies (EQEs) of 24.3% (at maximum), 23.9% (at 1000 cd m-2), and 18.7% (at 10 000 cd m-2), with the latter ranking the highest among the previously reported OLED devices employing deep-blue MR-TADF emitters.